A conventional shift range switchover control device for a transmission of a vehicle switches over a shift range by controlling a motor in correspondence to a shift range switchover request from a driver. In the shift range switchover device, an electronic control unit (ECU) controls the motor. When the motor is supplied with a current, the ECU and the motor both generate heat because of respective operations. When the driver makes shift range switchover requests repetitively, temperatures of the ECU and the motor rise in a relatively short time period. It is therefore necessary to estimate the temperatures of the ECU and the motor and control the motor so that the temperatures of the ECU and the motor do not exceed respective permissible heat generation temperatures, that is, respective overheat temperature limits. According to JP 5477437 (US 2013/0144478 A1: patent document 1), for example, the motor is prohibited from being driven to protect the ECU and the motor when an estimated temperature of the ECU or the motor reaches the permissible heat generation temperature.
When the driver turns on and off an ignition switch in the vehicle repetitively, the temperatures of the ECU and the motor also rise similarly to the case that the shift range is switched over repetitively. This is because initial driving control is performed to learn a relative position between a rotation position of a rotor and a current supply phase of the motor, which is necessary for the shift range switchover control, when power supply is started to the ECU. According to JP 5093624 (US 2011/0175563 A1: patent document 2), for example, even when the ignition switch is turned off, the power supply to the ECU is continued without immediately turning off a main relay until a predetermined prohibition period elapses. As a result, even when the driver repeats turning on and off the power supply in the prohibition period, normal motor control is performed without performing the initial driving control each time the power supply is started. Thus heat generation of the motor is reduced.
According to heat protection control and heat generation suppression control performed in patent document 1 and patent document 2, it is not possible to estimate temperatures when the power supply to the ECU is turned off. For this reason, a setting of motor temperature estimated at the time of starting the motor control when the power supply to the ECU is turned on again and a setting of main relay turn-off prohibition period after the turn-off of the ignition switch need be improved.
In patent document 1, the motor will not be protected properly in a case that the ignition switch is turned on again immediately after the ignition switch was turned off with the estimated motor temperature being close to the permissible heat generation temperature, if the motor temperature at the restart time is set to a product-mounting environment temperature.
In patent document 2, it is likely that the motor temperature cannot be estimated properly after the ignition switch is turned on again, if the motor temperature remains to be higher than the product-mounting environment temperature after elapse of the prohibition period. To avoid this problem, the prohibition period may be set longer, for example, to surely lower the motor temperature to the product-mounting environment temperature. With such a long prohibition period, when the driver does not make the shift range switchover request operations or turn on and off the ignition switch repetitively, the power supply to the ECU is continued even when the estimated motor temperature has already fallen to the product-mounting environment temperature. Since this causes discharging of a battery unnecessarily, the prohibition period need be set properly.